The present invention generally relates to the use of phase modulated spectroscopy (PMS) to to detect pathophysiological changes in a subject. More particularly, the present invention relates to methods and apparatuses for carrying out phase modulated spectroscopy. Still more particularly, the invention relates to improvements on the apparatuses disclosed in the above-referenced related applications.
The use of PMS to monitor the distribution of light pathlengths for detection of tissue absorption changes is discussed in the related applications. Further background is provided in Sevick, et al., "Analysis of Absorption, Scattering, and Hemoglobin Saturation Using Phase Modulation Spectroscopy," SPIE (1991), which is incorporated herein by reference. Briefly, the basis for using optical pathlength information to detect tissue absorption is that, as the absorption properties of a scattering medium increase (decrease), the probability of photons travelling long pathlengths within the medium decreases (increases), and therefore the mean optical pathlength travelled by the photons is reduced (increased).